(a) Field of the Invention
The present invention relates to styrene-butadiene latex, specifically a method of preparing latex for coating paper which shows excellent adhesion while maintaining printing property and comprised in a composition for coating paper.
(b) Description of the Related Art
In preparing paper for coating, recently the companies reduce the cost price through increase of coating speed, addition of a functional inorganic pigment and reduction of amount of binders. Specifically, in latex comprised in coating composition, mechanical stability and polymerization stability by small-particlization are very important factors, and adhesion becomes a very important one of printing properties.
A conventional coating method of paper with coating composition comprises transferring coating solution from an applicator-roll to surface of paper and removing excess coating solution by a blade or an air-knife. In this case, since the coating solution is subjected to high pressure, high shear fluidity and mechanical stability that is different from polymerization stability of latex are very important. Thus, as coating speed increases, such mechanical stability and high shear fluidity will be important and they will be important factors to determine the productivity of coating paper.
If the stability of latex is destructed under the high pressure, the coating solution around the applicator-roll is gummed up, which causes quality deterioration of paper as well as remarkable reduction of continuous product of paper.
In addition, in preparing latex for coating paper, since latex is used as a binder, a lot of scales are produced in a stirrer or inner side of a reactor. As latex is made to small particles, the coagulants increase. They act as impurities in the following process, which results in unevenness of material properties in the continuing process and decreases productivity because much time and effort will be needed to remove them.
Unlike the above bulk coagulant in the reactor, fine coagulum exists in the suspension. It tends to increase the amount as latex is made to small particles. The coagulum is screened after complete of polymerization and during preparing coating solution by screening. If there are many impurities, it is difficult to control products because it needs much time and cost. Also, a severe problem such as streak occurs on the surface of paper so that it deteriorates the quality of paper.
As latex is made to small particles and concentration of solid ratio increases gradually, it is important for latex to have polymerization stability. The polymerization stability relates to improvement of productivity and high-shear fluidity as well as increase of productivity. This also lessens unevenness of material properties in the continuous producing so that it is helpful to improve quality relatively.
Due to recent high-speed coating, it is required to have stability of latex and due to high speed printing tendency, it becomes important to have adhesion.
The above paper coating composition comprises a pigment, adhesive and other additives. The synthetic latex such as styrene-butadiene latex is mainly used as the adhesive. The adhesive significantly affects the quality of printing paper, specifically adhesion.
The important printing properties include adhesion (dry pick resistance), drying speed of ink, gloss, water resistance, and wet ink receptivity.
Due to high cost of latex of the coating composition and the tendency of high-speed printing, the importance of adhesion becomes high because the companies require reducing the amount of latex. That is, the coating paper should resist strong mechanical strength against surface of coating paper of pigments with as less amount of latex as possible so that it does not occur exfoliation from coating layer and falling off of pigment, thus, the coating paper can have clear printing appearance.
In addition, factors of styrene-butadiene latex affecting to adhesion of coating paper are glass transition temperature, particle size, composition of monomers and so on. Generally, adhesion shows high value at determined s gel content and the optimized gel content value depends on the composition of monomers.
Another important factor is a drying speed of ink. In the case of multi color printing, printing is done by four colors of blue, black, red, and yellow. As printing speed increases, time term to print the next color becomes short, lo thus it is required faster drying speed of ink. If ink is not well dried and goes to the next step, print mottle or post-smear can occur. The relationship of gel content and drying speed of ink affects film-formability according to gel content as well as swelling index showing amount of solvent which latex particle can contain.
Gloss is an important factor to increase commercial value and quality of printing paper. Gloss includes paper gloss of coating paper and printing gloss after printing. As the value of both of them increases, paper has good appearance.
In order to increase paper gloss, generally particle size of latex should increase or content of latex should decrease. However, those methods have a demerit of lowering adhesion. In order to increase printing gloss, air permeability should decrease so that it is required to hold solvent on the surface until having stable arrangement after printing. Thus, it needs to decrease drying speed of ink.
In addition, water resistance is also important printing property of offset printing. In the offset printing, water is used in the printing process. If water resistance(wet pick resistance)decreases, pigments can be exfoliated by applied strong physical strength during printing.
As like adhesion, water resistance and gel content have a relationship. That is, water resistance is the strongest at determined gel content. However, gel content that shows the strongest water resistance and gel content that shows the highest adhesion do not coincide. There is a tendency that water resistance shows the maximum point at lower gel content and adhesion shows the maximum point at higher gel content.
Another printing property required in the offset printing is wet ink receptivity. As described in the above, since water is used in the offset printing, if coating paper does not effectively absorb water when printing, ink that does not have compatibility with water will not adhere to coating paper, which results in low degree of printing. Generally, wet ink receptivity and water resistance are opposite properties so that it is difficult to increase both of them.
As above, it is very difficult to prepare latex that can provide coating paper with excellent printing properties and coating and printing circumstances become stricter.